1. Field of the Invention
The present invention relates to a double-side polishing process for semiconductor wafers, which are used in particular in industry for the fabrication of microelectronic components. The invention also relates to a device which is suitable for carrying out the process.
2. The Prior Art
Technological progress in the fabrication of microelectronic components, for example processors and memory components, requires the provision of single-crystal semiconductor wafers as a platform. These wafers have to satisfy increasingly stringent specifications. Specifications of this type relate to the crystal quality, and relate to the surface, in particular the front surface which is intended for component fabrication, and relate to the geometry and to the nano-topography of wafers of this type.
Conventional single-side polishing processes are no longer sufficient to produce semiconductor wafers which satisfy increasingly stringent geometry and nano-topography requirements. Moreover, the users of modern component processes increasingly demand not only a polished front surface on which the components are placed but also a polished back surface. This back surface binds fewer particles than, for example, an etched back surface. Therefore this back surface reduces the component failure rate caused by electric short circuits which are due to cross-contamination. For this reason, equipment and processes have been provided and developed further for the simultaneous polishing of front and back surfaces of the semiconductor wafer using what is known as the double-side polishing process. This equipment and these processes are nowadays being increasingly used in particular for the industrial manufacture of semiconductor wafers with diameters of 200 mm and 300 mm. The semiconductor wafers are moved in carriers made from metal or plastic, which carriers have suitably dimensioned cutouts. The carriers are moved over a path which is predetermined by the machine and process parameters between two rotating polishing plates. These plates are covered with polishing cloth, in the presence of a polishing abrasive, and as a result the wafers are polished, so that a high plane parallelism is achieved.
According to the prior art, which results, for example, from U.S. Pat. No. 4,974,370, the carriers are moved as follows. This is done either by means of an involute gear teeth, in which carrier gears and both outer and inner drive gears of the polishing machine come into engagement. Alternatively, this is done by pin wheel gears, in which case the carrier is surrounded by generally semicircular cutouts, in which pins of the outer and inner gears, belonging to drive gears, engage. During the polishing, the polishing abrasive flows continuously out of a stationary supply in the central axis of the polishing machine onto an open polishing-abrasive ring channel. This is attached to the upper polishing plate and therefore rotates, and from which it is passed, by means of hoses or tubes, through bores in the plate, to the semiconductor wafers which are to be polished.
As with single-side polishing processes, suitable polishing abrasives for double-side polishing are alkaline suspensions of abrasive substances, for example SiO2 colloids in combination with alkaline components in water. Polishing abrasives of this type and their production, as well as suitable supply systems, are described, for example, in DE 197 15 974 A1, DE 198 17 087 A1, EP 959 116 A2 and U.S. Pat. No. 6,027,669.
A double-side polishing process for achieving improved planarity is described in 199 05 737 C2. By way of example, carriers made from chromium steel, which form the subject matter of the German patent application which bears the reference number DE 100 23 002.4, are suitable for this purpose. To avoid damage to the edges during polishing, it has proven appropriate to line the cutouts in the carriers which are intended to receive the semiconductor wafer with plastic, for example as described in EP 208 315 B1. However, different factors of influence, for example coagulation, and as a result crystallization of the polishing abrasive cause crystallites to form in the open supply under the action of ambient air. In addition, abrasion of polishing-abrasive deposits and extremely small metal particles can be formed on the drive gears and constantly cause scratches on the polished semiconductor wafers. These scratches can only be removed to a certain extent by expensive further polishing and lead to increased levels of scrap.
A person skilled in the art is aware that the treatment of polishing cloths by means of brush plates to clean away particles can be carried out between two polishing runs. In the case of double-side polishing, the use of this process has had only limited success, since there is a continuous supply of particles from the polishing-abrasive ring channel and/or from the gears. Manual cleaning of these components generally only provides a remedy for a short time.
EP 787 562 B1 has described a double-side polishing process which is distinguished by the fact that at least part of the carrier mount and of the pin or gear drive are produced from a hard resin material. Although this process reduces the risk of scratches being formed by metal particles, it has no effect on the formation of scratches caused by encrusted polishing abrasive flaking or being rubbed off. WO 00/39841 describes the storage of the carriers for double-side polishing under water between the polishing runs. This is done in order to prevent polishing abrasive from drying on the carriers, and therefore to prevent the formation of scratches, but the same restriction as that referred to above applies.
The prior art has not hitherto disclosed any process for the double-side polishing of semiconductor wafers which ensures a constantly low scratch rate.
It is therefore an object of the present invention to provide a process for the double-side polishing of semiconductor wafers which ensures a constantly low scratch rate.
The above object is achieved by the invention which provides a process for producing semiconductor wafers by double-sided polishing between two rotating, upper and lower polishing plates, which are covered with polishing cloth, while an alkaline polishing abrasive with colloidal solid fractions is being supplied, the semiconductor wafers being guided by carriers which have circumferential teeth and are set in rotation by complementary outer and inner gears of the polishing machine, in which the following process steps are simultaneously carried out during the polishing of the semiconductor wafers:
(a) at least one of the two sets of gears of the polishing machine is at least from time to time sprayed with a liquid which substantially comprises water,
(b) the alkaline polishing abrasive is fed continuously to the semiconductor wafers in a closed supply device.
The present invention is further directed to a polishing machine for the double-sided polishing of semiconductor wafers, comprising an upper polishing plate and a lower polishing plate, each plate of which is covered with polishing cloth, carriers for receiving semiconductor wafers and a drive with external and internal gear teeth for turning the carriers, which machine has means for spraying at least one of the two sets of gears with a liquid and a closed system for supplying a polishing abrasive to the semiconductor wafers.
An essential feature of the invention is that, by preventing the polishing abrasive supplied from drying out, crystallization of the colloid, which leads to scratches being formed on the surfaces of the semiconductor wafers, is avoided. The fact that only the combination of the measures of (a) wetting of the outer and/or inner gear teeth of the polishing machine and (b) using a closed polishing-abrasive supply, which prevent the polishing abrasive from drying out, considerably reduces the scratch rate on the semiconductor wafers. This is unexpectedly surprising and was impossible to predict.
The starting material for the process is formed by using semiconductor wafers which are produced by sawing a semiconductor crystal, with rounded edges. The wafers are then subjected to one or more process steps such as lapping, grinding, etching and polishing. The end product of the process is double-side polished semiconductor wafers which, on account of a considerably reduced scratch rate, can be obtained in high yields on a large industrial scale and are therefore superior in terms of production costs to semiconductor wafers produced according to the prior art.
The process according to the invention can in principle be used to produce a body in wafer form which consists of a material which can be processed by the chemical mechanical double-side polishing process employed. Examples of such materials are silicon, silicon/germanium, silicon dioxide, silicon nitride and gallium arsenide. The use of silicon single crystal wafers is particularly preferred and forms the subject of the description which follows.
The silicon wafers which are produced by sawing up a silicon single crystal and rounding the edges can be subjected to various material-removing process steps before the double-side polishing process according to the invention is carried out. The purpose of these material-removing process steps is to improve the wafer geometry and to remove damaged surface layers and contamination. Suitable processes are lapping, grinding and etching. Wafers with polished surfaces can also be subjected to the process according to the invention. The preferred diameter of the silicon wafers which are to be polished is 150 to 450 mm.
A commercially available double-side polishing machine of suitable size can be used to carry out the polishing step according to the invention. The polishing machine substantially comprises a lower polishing plate, which can rotate freely in the horizontal plane, and an upper polishing plate, which can rotate freely in the horizontal plane, both of which plates are covered with a polishing cloth. This allows the double-sided abrasive polishing of silicon wafers with an alkaline polishing abrasive being supplied continuously. During the polishing, the silicon wafers are held on a geometric path which is determined by machine and process parameters. These wafers are held by carriers which have appropriately dimensioned cutouts for receiving these silicon wafers and have a thickness which is slightly less than that of the wafers.
The simultaneous use of at least three carriers is preferred. The simultaneous use of from four to six planar carriers made from stainless chromium steel is described in the German patent application which bears the reference numeral 100 23 002.4. Each carrier is loaded with at least three silicon wafers, and the edges of which are protected by polymer linings in the cutouts. This arrangement is particularly preferred. By way of example, it is possible, within the scope of the invention, to simultaneously polish 30 silicon wafers with a diameter of 200 mm (distributed over five carriers each holding six silicon wafers). It is also possible to polish simultaneously 15 silicon wafers with a diameter of 300 mm (distributed over five carriers each holding three silicon wafers).
The carriers are in contact with the polishing machine, for example by means of a pin wheel gear or an involute gear, via a rotating inner drive pin or gear ring and an outer drive pin or gear ring, which generally rotates in the opposite direction. As a result they are set in rotary motion between the two polishing plates. On account of the relative simplicity of exchanging pins which become worn through the constant action of the carrier flanks, the pin gear is preferred within the context of the invention in order to reduce the number of scratches. A further reason is that because of the optimized transmission of forces between the semicircular cutout of the carrier gear and the drive pins, which is optimized compared to that achieved with the sawtooth-like involute gear, the machine operates with fewer vibrations. Also it is therefore possible, by quicker rotation kinetics and a higher pressure, to achieve more advantageous material-removal rates during polishing, for example 0.8 to 1.5 mm/min. Drive pins which comprise a fixedly mounted pin and an exchangeable, secured, yet freely rotatable sleeve pushed over the pin, made from hardened VA steel, are particularly preferred.
Within the context of the invention, there is provision for the outer and/or inner drive ring of the polishing machine, which come into contact with the complementary gear teeth of the carriers, to be kept moist by spraying. This is done in order to counteract the deposition of polishing abrasive which dries out. This prevents the mechanical action of the carrier edges from detaching crystallites, for example of SiO2, if a corresponding polishing abrasive is used. This prevents the crystallites from being carried between the upper and lower polishing plates, so that they could cause scratches on the silicon wafers. The spraying is preferably carried out continuously with ultrapure water, although it may also be stopped for brief periods during the polishing. The quantity of water supplied is such that there is no significant dilution of the polishing abrasive supplied as a result of the water being introduced into the polishing machine via the carriers. The provision of spray nozzles which are directed onto the gears or pin rings from above or at an oblique angle depends on the size of the polishing machine. Overall at least one nozzle is to be provided. By way of example, for a double-side polishing machine with a plate diameter of about 2 m, one to three nozzles are to be provided on the inside and two to five nozzles are to be provided on the outside.
Within the context of the invention, the polishing abrasive is supplied via a closed supply system. Compared to the open polishing-abrasive channels used in the prior art, a system of this type has the advantage that there are no significant encrustations of crystallized polishing abrasive which are washed between the polishing plates and may cause scratches. It is possible to use a closed polishing-abrasive ring channel which is acted on by pressure, and which is attached to the upper polishing plate. Therefore it rotates with the upper polishing plate and allows a forced transfer of the polishing abrasive, for example through hoses or tubes, through the upper polishing plate to the location of polishing. A design of this type is complex, since the polishing abrasive from the feed line, in the unmoving central part of the polishing machine, can only be transferred into the ring channel by rotary passages of complicated design.
A closed but unpressurized ring channel represents a simpler and therefore preferred design. In this case the polishing abrasive from the machine supply flows into the ring channel through the application of pressure. It then flows out of the ring channel and between the polishing plates on account of the force of gravity and centrifugal force. The ring channel, which rotates with the upper polishing plate, is covered by a cover. This cover is likewise in the form of a ring, but unlike the ring channel does not rotate and comes into contact particularly preferably via a liquid seal, which is particularly preferably filled with water. It is possible for the interior of the ring channel to be continuously or discontinuously sprayed with a liquid, preferably water. This spraying is by means of at least one nozzle which is attached, for example, to the underside of the non-rotating cover, although this is not absolutely imperative within the context of the invention.
Within the context of the statements which have been made above, the double-side polishing step is preferably carried out in the manner which is known to the person skilled in the art. Polishing preferably uses a commercially available polyurethane polishing cloth with a hardness of between 60 and 90 (Shore A), which particularly preferably has incorporated polyester fibers. For optimum distribution of the polishing abrasive between the upper and lower polishing plates, it is particularly preferable for at least the upper polishing cloth to be provided with a network of channels. These channels may, for example, be arranged in the manner of a chessboard, with a segment size of from 5 mmxc3x975 mm to 50 mmxc3x9750 mm and a channel width and depth of from 0.5 to 2 mm. In the case of the polishing of silicon wafers, the continuous supply of a polishing abrasive with a pH of from 9 to 12, is set by the additions of alkali. This polishing abrasive comprises a colloid which particularly preferably comprises 0.5 to 10% by weight of SiO2 in water. It may contain silica which, for example, is precipitated from water glass and/or produced pyrolytically from Si(OH)4, and is recommended. After a certain number of polishing runs, for example after one to six polishing runs, it is recommended for the upper and lower polishing cloths to be brushed with brush plates. They are fitted instead of the carriers, for a period of, for example, 2 to 10 min while ultrapure water is being supplied, in order to prevent vitrification of the surface and to remove dirt particles.
The weight percent of SiO2 in water is based upon the total weight of the polishing abrasive.
After the polishing has ended, any adhering polishing abrasive is cleaned off the silicon wafers, and the latter are dried and then examined for surface scratches under sharply focused light. Depending on their further purpose, it may be necessary for the wafer front surface to be subjected to surface polishing according to the prior art. For example this may be done by using a soft polishing cloth with the aid of an alkaline polishing abrasive based on SiO2. It could also be desirable to apply an epitaxial coating of a semiconductor material, for example likewise of silicon. This is possible immediately after the double-side polishing has been carried out in accordance with the invention or after an additional surface polishing step, without problems.
The double-side polishing process according to the invention makes it possible to produce semiconductor wafers from silicon which satisfy the requirements for the fabrication of modern semiconductor components. In this process, when large quantities of wafers are produced, a reject rate caused by scratches of only 0.5 to 2% is observed if double-sided polishing is carried out with spraying of the outer and/or inner drive gear ring and with a closed supply of polishing abrasive. This leads to cost savings over processes of the prior art. A process according to the prior art without the inventive features, by contrast, leads to a scrap rate caused by scratches which is of the order of magnitude of 5% to 20%. This leads to higher production costs, for example through the need to reject the wafers or for complex further polishing without the guarantee of success and with the risk of an adverse effect on other quality parameters.